As well known by people involved in the art, chromatographic systems rely on the use of valves to allow reproducible sample introduction and various column switching schemes.
For the last forty years, many people have designed diaphragm valves for chromatography. Such diaphragm valves have been used in many commercially available gas chromatographs. They are apt to be integrated more easily in a gas chromatograph due to their physical size and since the actuator is embedded in the valve itself. These characteristics make them attractive for gas chromatograph manufacturers.
Referring to FIG. 1 (PRIOR ART), there is shown an example of a typical diaphragm-sealed valve as known in the art. The valve 1 is provided with a top block 2 having an interface 4 and a plurality of ports 6. Each of the ports 6 opens at the interface 4 and has an inclined thread passage 8 to connect various analytical fitting and tubing (not shown). At the bottom of the inclined thread passage 8, there is a conduit 10 extending in the top block 2 and opening at the interface 4. The ports 6 are arranged on a circular line on the interface 4 of the top block 2. The interface 4 is advantageously flat and polished to minimize leaks between ports and from the ambient atmosphere. The valve 1 is also provided with a bottom block 12 and a diaphragm 14, which is generally made of polyimide, Teflon or other polymer material. The diaphragm 14 is positioned between the top block interface 4 and the bottom block 12, and has a recess therein extending along the circular line formed by the ports 6 and biased away from the interface 4 of the top block 2. The recess 18 in the diaphragm 14 sits in a matching recess 20 made in the bottom block 12, thereby allowing some clearance for fluid circulation between adjacent ports 6.
The valve 1 is also provided with a plurality of plungers 16 mounted in the bottom block 12, each being respectively arranged to be able to compress the diaphragm 14 against the top block 2 at a position located between two of the ports 6. Preferably, as illustrated, when the valve is at rest, three plungers 16 are up while the other three are down. When the plungers are up, they compress the diaphragm 14 against the top block 2 and close the conduits made by the diaphragm recess 18, so that fluid circulation is blocked. The bottom block 12 keeps the plungers 16 and the actuating mechanism in position.
The performance of valves of the type shown in FIG. 1 is generally poor. The leak rate from port to port is too high for most applications and thus limits the system performance. Moreover, the pressure drop on the valve's ports differs from port to port, causing pressure and flow variations in the system. This causes detrimental effects on column performance and detector baseline. Furthermore, many valve designs allow for unacceptable inboard contamination.
As explained above, in the valve of FIG. 1, the sealing of the fluid circulation path between two ports 6 relies simply on the surface of the plunger defining the area that presses the diaphragm recess 18 on the interface 4. This technique imposes tight restrictions on the surface finish and flatness, and on the length of the plungers. Any scratch on the interface 4 or imperfection of the diaphragm 14 will generate leaks. Moreover, the length of all plungers must be the same. Any difference in their lengths will result in leaks, since a shorter plunger will not properly compress the diaphragm against the interface 4.
Several variations of the general concept of the valve of FIG. 1 are known in the art. The main differences relate to the location of the bottom block recess 20. In the past, this recess 20 or its equivalent was located internally in the top block 2, or on its interface 4. U.S. Pat. Nos. 3,111,849; 3,198,018; 3,545,491; 3,633,426 and 4,112,766, which were granted to the same inventors, illustrate valves provided with such recesses. However, as reported in a more recent valve brochure specification entitled “Applied Automation Company, series 11 diaphragm valve” (now commercialized by Siemens), this method is no longer used because of a too high cold flow. Cold flow is also often referred to as cross port flow leak. The latest design from the same group, which was commercialized, uses a flat and polished interface 4 on the top block 2 and a recess 20 in the bottom block 12. In this design, the diaphragm 14 has no recess. Moreover, in order to reduce the cold flow, it was also envisaged to use two diaphragms. U.S. Pat. No. 3,111,849 teaches the use of a “cushion” diaphragm to allegedly compensate for any slight non-parallelism or length difference of plungers. Other attempts have also been made to correct the non-parallelism, as disclosed in U.S. Pat. Nos. 3,376,894; 3,545,491 and 3,633,426, wherein the use of solid plungers has been replaced with the use of small steel balls.
The above-mentioned concern about plunger length has also been taken into consideration in U.S. Pat. No. 6,202,698, granted to Valco Company, which suggests the use of plungers made of softer material. This allows tolerance reduction for the length of such plungers. However, such a design will still result in an important leak rate between ports since the pressure from the plungers is not equal on the diaphragm.
Other attempts have been made in the past to eliminate problems caused by plunger tolerance variations. U.S. Pat. No. 3,139,755 discloses a valve devoid of plungers, hydraulic pressure being used instead. However, an auxiliary source of pressure must be used, since no pneumatic amplification of the pneumatic actuating mechanism is performed. Another design is disclosed in U.S. Pat. No. 3,085,440. In this valve, the diaphragm has been replaced by an O-ring.
In brief, in view of the previously mentioned patents, it can be seen that many attempts have been made to fix cross port leaks problems and outboard or inboard contamination. All of the proposed designs are quite similar with regard to sealing mechanisms, and have the same drawbacks.
There is therefore a need for an improved diaphragm-sealed valve.